You might need compression
software like WinZip (www.winzip.com)
to decompress the file. It's also possible that the file you're
trying to open is either the wrong version or requires software
that you don't have. Your best bet is to e-mail the sender
back and ask what type of file they sent you. Even better,
if it's just text, have them include it in the body of the
e-mail rather than sending it as an attachment.